As a means of receiving quadrature phase shift keyed (QPSK) signals for coherent optical communication etc., waveguide-integrated optical receivers consisting of a hybrid optical waveguide integrated with a photodiode are promising from the viewpoint of size reduction and assembly cost reduction. A hybrid optical waveguide converts phase-modulated optical signal into intensity-modulated optical signal. A multimode interference (MMI) waveguide etc. is commonly used as hybrid optical waveguide.
A waveguide and a photodiode formed on a substrate are coupled with each other, for instance, using a butt joint structure. In the butt joint structure, an optical signal that has passed through the waveguide directly enters the absorption layer of the photodiode. Accordingly, a high light absorption efficiency can be maintained even if the photodiode size is reduced with respect to the propagate direction of the optical signal.
In the butt joint structure, on the other hand, a large part of the incident light is absorbed in a very short region that has a length of several micrometers from the light-incident end (input end) of the photodiode. Accordingly, the photo carrier density tends to increase in the neighborhood of the light-incident end. If the incident light is high in intensity, photo carriers tend to be accumulated in the absorption layer of the photodiode. The operating bandwidth of the photodiode becomes narrow because the electric field generated by the accumulated photo carriers acts to decrease the intensity of the applied electric field.
There has been a proposed technique that uses a MMI waveguide disposed between an input waveguide and a photodiode to reduce the peak with respect to the width direction of the light intensity distribution. If a single mode optical signal enters the MMI waveguide, two or more peaks will appear in the light intensity distribution at the light-emitting end (output end) of the MMI waveguide. Accordingly, the light intensity per peak decreases. Therefore, the local rise in photo carrier density caused by the incident light can be suppressed.
Light waveguide devices in which an optical signal propagating through a single mode optical waveguide is divided equally by a tapered waveguide into two or more output ports have also been known generally as an optical waveguide apparatus having the same function as the MMI waveguide.